1. Technical Field of the Invention
The present invention relates to an optical encoder for detecting a position in movement, in rotation and so on.
2. Description of the Related Art
In an inkjet printer for example, printing is performed while moving the printing head. Therefore, it is necessary to accurately detect the position of the printing head in the movement. In such a case as this, use of an optical encoder makes possible to accurately detect the position of the moving printing head.
An example of the optical encoder is disclosed in JP-A-61-292016 (corresponding U.S. Pat. No. 4,691,101). This optical encoder comprises, as shown in FIG. 24, an optical unit 103 including a light emitter 101 and a light receiver 102, and a light controlling member 105 disposed between the light emitter 101 and the light receiver 102. The optical unit 103 and the light controlling member 105 can move relative to each other longitudinally of the light controlling member 105. For example, if the optical member 103 is mounted on the printing head of the inkjet printer and the light controlling member 105 is fixed to a case of the inkjet printer, the optical unit 103 moves along the light controlling member 105 when the printing head moves.
The light controlling member 105 is made of a ribbon-like resin film for example, and as shown in FIG. 25, formed with a plurality of transparent portions 106 and nontransparent portions 107 alternating with each other. All of the transparent portions 106 have a same length longitudinally of the light controlling member 105. All of the nontransparent portions 107 have a same length longitudinally of the light controlling member 105. Further, the length of the transparent portion 106 longitudinally of the light controlling member 105 and the length of the nontransparent portion 107 longitudinally of the light controlling member 105 are equal to each other. In other words, in a pairs of adjacent transparent portion 106 and nontransparent portion 105, a length L as a sum of the length of the transparent portion 106 longitudinally of the light controlling member 105 and the length of the nontransparent portion 107 longitudinally of the light controlling member 105 is constant in any pair of the transparent portion 106 and the nontransparent portion 107.
The light receiver 102 includes, as shown in FIG. 26, a photodiode group 111 made of four photodiodes 111a-111d. These four photodiodes 111a-111d are arranged close to each other in the direction of the relative movement between the optical unit 103 and the light controlling member 105. All of the photodiodes 111a -111d have a same length in the direction of the arrangement, and a total of the four lengths is K. In other words, a length of the photodiode group 111 in the direction of the relative movement between the optical unit 103 and the light controlling member 105 is K. In the above arrangement, K and L are exactly equal to each other or generally equal to each other within a manufacturing error. It should be noted here that there is a slight gap between each pair of adjacent photodiodes 111a-111d due to technical reasons of manufacture, but these gaps are not illustrated in FIG. 26.
The photodiodes 111a-111d have output terminals connected with input terminals of four adders 113-116 as shown in FIG. 27. Specifically, the input terminals of the adder 113 are connected with the output terminals of the photodiodes 111a, 111b. The input terminals of the adder 114 are connected with the output terminals of the photodiodes 111c, 111d. The input terminals of the adder 115 are connected with the output terminals of the photodiodes 111b, 111c. The input terminals of the adder 116 are connected with the output terminals of the photodiodes 111a, 11d. The adders 113-116 have output terminals connected with input terminals of two comparators 118, 119. Specifically, the input terminals of the comparator 118 are connected with the output terminals of the adders 113, 114. The input terminals of the comparator 119 are connected with the output terminals of the adders 115, 116.
If the light controlling member 105 moves in a direction indicated by Arrow A in FIG. 25 at a constant speed, or if the optical unit 103 moves in a direction opposite to the direction indicated by Arrow A at a constant speed, the photodiodes 111a-111d give output signals as shown in FIG. 28.
Therefore, outputs from the adders 113, 114 and the comparator 118 are as shown in FIG. 29. It should be noted here that the comparator 118 outputs a high-level signal if the output from the adder 113 is greater than the output from the adder 114.
Further, outputs from the adders 115, 116 and the comparator 119 are as shown in FIG. 30. The comparator 119 outputs a high-level signal if the output from the adder 115 is greater than the output from the adder 116.
As described, in the prior art optical encoder, the photodiodes 111a-111d are so manufactured that the photodiode group 111 has the dimension K that is equal to the dimension L as the sum of one transparent portion 106 and one nontransparent portion 107, thereby obtaining from the comparator 118 and the comparator 119 the output signals having a phase shift of a quarter of the period.
However, according to the prior art optical encoder, in order to make the dimension K and the dimension L as exactly the same as possible, the photodiodes 111a-111d must be manufactured at a high accuracy, leading to a problem of increased cost of manufacture. Further, at an occasion when the dimension L is altered for improved detection accuracy, or for manufacture of a plurality of kinds of the product each having a different value in the dimension L, it is necessary to differentiate the size of the photodiode group 111 for each specific value of the dimension L in the manufacture of the optical unit 113, leading again to the problem of increased cost of manufacture. Further, even if the comparator 118 and the comparator 119 give output signals having the phase shift Of a quarter of the period, advantage of receiving such signals can only be fully enjoyed in a special application. In a general application such as position detection of the printing head in an inkjet printer, the phase shift between the output from the comparator 118 and the output from the comparator 119 may not necessarily be a quarter of the period, but rather it is only necessary that the output from the comparator 118 and the output from the comparator 119 are comparable so as to discern the direction of the relative movement between the optical unit 103 and the light controlling member 105.
Further, according to the above prior art optical encoder, the photodiodes 111a-111d are arranged in a line in the direction of the relative movement between the optical unit 103 and the light controlling member 105. With this arrangement, if the length L of the pair of transparent portion 106 and nontransparent portion 105 is small, the outputs from the photodiodes 111a-111d are small, which leads to deterioration in S/N ratio and a problem to detect accurately.
Specifically, in order to improve detection accuracy of the optical encoder, the length L of the pair of transparent portion 106 and nontransparent portion 107 must be made small, which obviously means the length of the array of the photodiodes 111a-111d must be small. However, due to technical reasons in manufacture, there is unavoidably a gap or a region of low sensitivity between each adjacent pair of the photodiodes 111a-111d. For this reason, if the length of the array of the photodiodes 111a-111d is made small, ratio of the low-sensitivity region to the high-sensitivity region increases, causing a sharp drop in the output from the photodiodes 111a -111d. As a result, the S/N ratio of the output signals from the photodiodes 111a-111d decreases, leading to inability to detect even if the signals are amplified. Therefore, the original objective, which is improvement in the detection accuracy, cannot be achieved.
Further, according to the prior art optical encoder, a large number of photodiodes 111a-111d are used to make the photodiode group 111. This has been another cause of the problem of increased cost of manufacture.
Specifically, even if the four photodiodes 111a-111d are used in the photodiode group 111, and output signals having the quarter phase shift are obtained from the comparator 118 and the comparator 119, advantage of using such signals can be fully enjoyed only in limited applications as has been described earlier.
It should be noted also that there is another prior art optical encoder in which the photodiode group is made of six photodiodes. Again, in this case, when the encoder is applied to a general purpose such as in the inkjet printer, outputs from only two comparators out of three are used for discerning the direction of relative movement between the optical unit and the light controlling member.
An object of the present invention to provide an optical encoder which can be manufactured at a favorably low cost.
Another object of the present invention is to provide an optical encoder having a favorably improved detection accuracy.
According to a first aspect of the present invention, there is provided an optical encoder comprising: an optical unit including a light emitter for emitting light and a light receiver for receiving the light from the light emitter; and a light controlling member including a plurality of transparent portions for passing the light from the light emitter and a plurality of nontransparent portions disposed alternately with the transparent portions for blocking the light from the light emitter. The transparent and the nontransparent portions are so arranged that any pair of adjacent transparent portion and nontransparent portion has a constant length in a direction of the adjacency. The optical unit and the light controlling member are movable relative to each other in a direction of arranging the transparent and the nontransparent portions. The light receiver is provided with one or any greater number of light receiver groups each including a plurality of adjacent light receiving elements arranged in a direction of the relative movement between the optical unit and the light controlling member. Each light receiver group has, in said direction of the relative movement, a length which is unequal to said constant length of the adjacent transparent portion and nontransparent portion.
According to a preferred embodiment, each of the light receiving elements in each light receiver group has a same length in a direction of the arranging the light receiving elements. Further, the length of each light receiver group in said direction of the relative movement is greater than a sum of said constant length of the adjacent transparent and nontransparent portions and a length of one light receiving element measured in the direction of arranging the light receiving elements.
According to another preferred embodiment, the light receiver groups are disposed at a predetermined pitch, and the predetermined pitch is equal to or a multiple of said constant length of the adjacent transparent and nontransparent portions.
According to a second aspect of the present invention, there is provided an optical encoder comprising: an optical unit including a light emitter for emitting light and a light receiver for receiving the light from the light emitter; and a light controlling member including a plurality of transparent portions for passing the light from the light emitter and a plurality of nontransparent portions disposed alternately with the transparent portions for blocking the light from the light emitter. The transparent and the nontransparent portions are so arranged that any pair of adjacent transparent portion and nontransparent portion has a constant length in a direction of the adjacency. The optical unit and the light controlling member are movable relative to each other in a direction of arranging the transparent and nontransparent portions. The light receiver is provided with one or any greater number of light receiver groups each including a plurality of adjacent light receiving elements arranged in a direction of the relative movement between the optical unit and the light controlling member. Each light receiver group has, in said direction of the relative movement, a length which is generally equal to said constant length of the adjacent transparent portion and nontransparent portion. The light receiving elements in each light receiver group are arranged in one line and adjacent line both extending in said direction of the relative movement. Said lines are spaced from each other in a direction perpendicular to said direction of the relative movement. The light receiving elements in said one line are offset from the light receiving elements in said adjacent line in said direction of the relative movement.
According to a preferred embodiment, each light receiver group includes four light receiving elements identical in shape and size. Two of the light receiving elements are arranged in said one line, while the other two light receiving elements are arranged in said adjacent line. The two light receiving elements in said one line are offset, in said direction of the relative movement, from the other two light receiving elements in said adjacent line by half a predetermined pitch at which the light receiving elements in each line are arranged.
According to another preferred embodiment, more than one light receiver group is arranged in said direction of the relative movement.
According to a third aspect of the present invention, there is provided an optical encoder comprising: an optical unit including a light emitter for emitting light and a light receiver for receiving the light from the light emitter; and a light controlling member including a plurality of transparent portions for passing the light from the light emitter and a plurality of nontransparent portions disposed alternately with the transparent portions for blocking the light from the light emitter. The transparent and the nontransparent portions are so arranged that any pair of adjacent transparent portion and nontransparent portion has a constant length in a direction of the adjacency. The optical unit and the light controlling member are movable relative to each other in a direction of arranging the transparent and the nontransparent portions. The light receiver is provided with one or any greater number of light receiver groups each including a plurality of adjacent light receiving elements arranged in a direction of the relative movement between the optical unit and the light controlling member. Each light receiver group includes three or greater odd number of light receiving elements. Said direction of the relative movement is discerned on a basis of both an output from one of the light receiving elements and a comparison between outputs from two of the remaining light receiving elements.
According to a preferred embodiment, each light receiver group includes three light receiving elements.
According to another preferred embodiment, each light receiver group has, in said direction of the relative movement, a length which is generally equal to said constant length of the adjacent transparent and nontransparent portions.
According to still another preferred embodiment, the odd number of light receiving elements except one of them have a same length in a direction of arranging the light receiving elements.
According to still another preferred embodiment, the light receiver groups are disposed at a predetermined pitch, and the predetermined pitch is equal to or a multiple of said constant length of the adjacent transparent and nontransparent portions.
Other characteristics and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings.